The present invention relates to a method for adjusting a photosensitive waveguide to have a desired stabilized optical path length. More specifically, the present invention provides a commercially feasible process for adjusting the optical path length of interferometer waveguide devices.
Optical path length is the distance light travels in a medium scaled by the refractive index n of the medium. The refractive index determines the speed v of an optical signal in the medium by the following equation,                     v        =                  c          n                                    (        1        )            
where c is the speed of light in vacuum. Optical path length (OPL) or xcex94 is defined by the following equation:
xcex94=nLxe2x80x83xe2x80x83(2)
where L is the physical length of the medium. As may be appreciated from equations (1) and (2) above, the OPL of an optical waveguide may be lengthened by increasing the index of refraction of the medium or shortened by decreasing the index of refraction. Precise control of the OPL of an optical waveguide segment or an optical waveguide device becomes a crucial issue when precise timing and synchronization of signals is needed or adjustment of the phase of an optical signal in relation to another is required.
The relative phase difference, "PHgr", between light beams at two locations, expressed as a fraction of an optical cycle, is
"PHgr"=xcex81xe2x88x92xcex82(mod 2xcfx80)xe2x80x83xe2x80x83(3)
where xcex81 is the wave phase of the first beam at the first location and xcex82 is the phase of the second beam at the second location. In some devices such as interferometers, one is interested in the phase difference between two beams at the same location. The phase difference is related to the difference in optical path length that the two beams have traveled. If the two light beams, beam 1 and beam 2, started at the same location with the same phase (as in interferometers), then                                           θ            1                    -                      θ            2                          =                                            2              ⁢              π                        λ                    ⁢                      (                                          Δ                1                            -                              Δ                2                                      )                                              (        4        )            
where xcex is the light wavelength in a vacuum and xcex941 and xcex942 are the optical path lengths that beam 1 and beam 2 traveled, respectively.
An interferometric device may be defined as an optical instrument that splits and then recombines a light beam, causing the recombined beams to interfere with one another. FIG. 1 illustrates the structure of a common interferometric device, a fiber Mach-Zehnder interferometer 10. The fiber Mach-Zehnder interferometer 10 includes a first input port 11, a second input port 12, a first leg 14, a second leg 16, a first output port 22, a second output port 24, a first coupler 26, and a second coupler 28. The terms input port and output port are relative, depending on the optical path length of each leg and the use of the device. Also, since the device is symmetric, the orientation of the device may be reversed.
A light signal enters either one of the two input ports, in the illustrated example of FIG. 1 through port 12. The light signal is then split into two component beams at the first coupler 26. The split beams travel independently through the two legs 14 and 16 of the interferometer. The two beams are recombined at the second coupler 28.
In most cases, it is desired to control the phase difference between the two beams at the recombination point, coupler 28. By making the phase difference equal to m*xcfx80 (where m is an integer) at this point, the input power may be made to exit at mostly either one or the other output port. As shown in equation 4, this phase difference is related to the OPL difference between the interferometer legs, which may be adjusted by changing the refractive index along a portion of a leg.
One way of using the interferometer as an optical add/drop multiplexer is to add Bragg gratings into the legs of the device. The interferometer 10 may include, as illustrated in FIGS. 2 and 3, an optional first Bragg grating 18 in the first leg and an optional second Bragg grating 20 in the second leg. As illustrated in FIGS. 2 and 3, a Mach-Zehnder add/drop device may be used to insert or remove a specific wavelength from an optical signal. FIG. 2 illustrates a Mach-Zehnder having gratings 18 and 20 that reflect a signal of a specific wavelength, xcex4, which is removed or dropped out of port 12. The remaining wavelengths exit through the first output port 22. FIG. 3 illustrates the opposite function, where a signal of a certain wavelength, xcex4, is inserted or added through port 24 and the recombined multiple wavelength signal exits through port 22. A description of the manufacture and use of couplers and of wavelength selective optical devices may be found in U.S. Pat. No. 4,900,119, relevant portions of which are incorporated herein by reference. The OPL difference between the legs in this add/drop device is to be properly set such that the two beams propagating through each leg of the device will recombine at the couplers 26 and 28 with the desired phase difference.
It is apparent that even a relatively small difference in optical path lengths between the two legs of the interferometer may change the performance of the device. For instance, axcx9c5xc2x0 error in phase difference between the two interferometer beams at the recombination coupler 28 may cause xcx9c5% of the input energy to exit ports that it shouldn""t exit, severely degrading device performance. Accordingly, in applications such as the above-described Mach-Zehnder device 10, it is important to adjust precisely the OPL difference between the device legs to control the manner in which the energy exits from the device. This is known as xe2x80x9coptical trimmingxe2x80x9d or xe2x80x9ctrimmingxe2x80x9d.
FIG. 4 illustrates common fabrication steps for creating optical fiber Mach-Zehnder devices. As seen in Step 1, the basic structure of a Mach-Zehnder interferometer is accomplished by fusing at two locations two lengths of optical fiber together until the cores are in close proximity. For some devices, it is considered important that the OPL of the two middle sections, the legs of the device, be about the same after the device is fused.
The resulting device, as seen in Step 2, may then be hydrogen loaded to increase the photosensitivity of the optical fibers. Methods for hydrogen loading optical fibers are discussed, for example, in U.S. Pat. Nos. 5,235,659 and 5,287,427 and in co-pending commonly assigned U.S. Application entitled xe2x80x9cACCELERATED METHOD FOR INCREASING THE PHOTOSENSITIVITY OF A GLASSY MATERIALxe2x80x9d, U.S. Ser. No. 09/616,117, filed Jul. 14, 2000, which is hereby incorporated by reference. Other methods for hydrogen loading an optical fiber are discussed in the relevant literature. Alternatively, increasing the photosensitivity of the fiber using doping or other methods known in the art may help eliminate Step 2.
Step 3 comprises writing a grating 18 and 20 into each one of the legs of the Mach-Zehnder interferometer. The step of writing a grating is usually achieved by exposing the photosensitive fiber to a pattern of actinic radiation. The pattern may be achieved in several ways, such as with a phase mask or a holographic approach known in the art. At this point of the manufacturing process, the OPL of the Mach-Zehnder interferometer legs will likely need adjusting, for even if the original OPLs were set properly, minute differences between the inscriptions of each grating into the legs would generally cause the device performance to degrade from the desired parameters. Device performance may degrade because the optical phase difference between the beams may change with the OPL difference and cause the balance of the input energy that exits from ports to change. Accordingly, one must adjust the OPL of at least one of the legs of the device to achieve a desired device operation. Traditionally this is attempted, as illustrated in Step 4, by changing the refractive index of regions 40 of the legs by exposing them to localized fringeless ultraviolet radiation. The UV exposure increases the refractive index of the exposed portions, lengthening the OPL of that exposed region. The exposure is done repeatedly while monitoring the light signal exiting one or more output ports, until the desired device performance is achieved.
When H2-loading is used to increase the photosensitivity of the exposed regions 40, the traditional OPL adjustment is complicated, since hydrogen is typically saturated throughout the device, including the entire length of the interferometer legs and the couplers. The presence of hydrogen in the couplers 26 and 28 changes the coupler performance and thus overall device performance, making OPL adjustment difficult. The presence of hydrogen in the interferometer legs also may change the refractive index of the legs and thus their OPL. As hydrogen desorbs from the device, with the passage of time and the effect of temperature, the coupler performance returns to that prior to hydrogen loading and the OPLs of the legs change. Adjustments of OPL made while the device was saturated with hydrogen may not be sufficient to ensure that the device will operate as desired after all the hydrogen desorbs.
A major shortcoming of the traditional OPL adjustment process is that the refractive index perturbations induced by ultraviolet-exposure to regions 40 are unstable, i.e. the perturbations will change with time and temperature. A device is considered xe2x80x9cstablexe2x80x9d if it can operate at a maximum operating temperature over a desired operating period of time without the device performance degrading beyond acceptable operating parameters. A typical maximum operating temperature that is specified commonly in the optical communication industry is 85xc2x0 C. An xe2x80x9cannealingxe2x80x9d process, in which the optical device is subject to temperatures much higher than the maximum operating temperature for a period of time, has been found to stabilize some optical devices, especially those manufactured by employing the phenomena of photosensitivity. When the Mach-Zehnder add/drop element is annealed, the induced index perturbations in both the gratings 18 and 20 and the trim regions 40 decrease as illustrated in step 5 of FIG. 4, upsetting the OPL adjustments of the device.
Because the OPL of the interferometer legs vary uncontrollably during several manufacturing process steps, current traditional manufacture of Mach-Zehnder devices is plagued by low manufacturing yields (and accordingly higher costs and lower production efficiency). Similar problems occur when attempting to reach precisely a desired optical path length in any length of photosensitive fiber when annealing still needs to be performed.
Although a particular fiber Mach-Zehnder add/drop device has been used to illustrate the problem with present methods of adjusting OPL in waveguide devices, one skilled in the art will recognize that these OPL adjustment difficulties exist when manufacturing other waveguide devices that rely on precise OPLs to operate properly. For example, one would encounter these difficulties when making other waveguide devices, such as planar waveguide interferometers, optical ring resonators, etalons, and Michelson interferometers.
The need remains for a reliable and accurate method for manufacturing photosensitive waveguide devices having a precisely adjusted optical path length.
The present invention relates to a method for adjusting a photosensitive optical waveguide to a desired stabilized optical path length and to devices manufactured in accordance with that method. In an exemplary embodiment of the method, exposing at least a first portion of the waveguide to actinic radiation increases the optical path length of a photosensitive optical waveguide by more than is needed. The exposure creates an induced refractive index increase on the exposed first portion. The waveguide is then subjected to an annealing cycle, which stabilizes the waveguide. After the step of stabilizing the waveguide, the optical path length is adjusted by subjecting at least a part of the exposed first portion of the waveguide to localized heating at a temperature higher than the maximum operating temperature of the device and sufficient to alter the induced index change until the desired optical path length is achieved. Applying heat to select portions of the device after it has been stabilized performs the adjustment process.
In a particular embodiment of the present invention, the method comprises the step of changing the optical path length of the photosensitive optical waveguide by exposing at least a first portion of the waveguide to actinic radiation and creating an induced refractive index change in the exposed first portion. The waveguide is subjected to an annealing cycle that stabilizes the waveguide. After the step of stabilizing the waveguide, the optical path length is adjusted by subjecting at least a selected part of the exposed first portion of the waveguide to a localized heating sufficient to change the refractive index at the selected part until the desired optical path length is achieved.
The waveguide may be a silica glass optical fiber, a planar waveguide, or other suitable waveguides. The waveguide may be treated to increase its photosensitivity, such as by placing it in a hydrogen-containing environment.
The method may further include the step of writing an optical grating in a second portion of the waveguide. The step of writing an optical grating may occur before the step of stabilizing the waveguide. The step of stabilizing the waveguide includes heating the waveguide to a first temperature to stabilize the device and the step of adjusting the optical path length includes heating at least a part of the exposed first portion to a second temperature, where the second temperature is greater than the first temperature.
In another embodiment, the step of stabilizing the waveguide includes heating the waveguide to a first temperature to stabilize the device and the step of adjusting the optical path length includes heating at least a part of the exposed first portion until the desired optical path length is achieved. The step of adjusting the optical path length may further comprise contemporaneously monitoring the optical path length of the waveguide during the localized heating exposure and terminating the exposure when the desired optical path length is reached. The heating exposure may be accomplished using a CO2 laser or another localized source of heat.
The method of the present invention may be applied in a variety of optical components. In an exemplary embodiment, the waveguide is a first leg of an interferometric device having at least a second leg and the step of adjusting the induced index change comprises adjusting the optical path length difference between the first leg and the second leg. The interferometer may be a Michelson, Mach-Zehnder, Sagnac, and Fabry-Perot interferometers, or other type of interferometer.
In a specific embodiment, the present invention yields an optical waveguide interferometric device having a first and a second output beams. The interferometric device has at least two interferometer legs at least one first leg having a photosensitive waveguide; an optical recombination point optically coupled to the at least one interferometer leg; and a portion of the at least one first leg having a refractive index perturbation larger by 10xe2x88x925 than surrounding waveguide material. The index perturbation is stabilized to the extent that the optical path length of the first leg only changes by an amount that causes a phase difference between the first and second output interferometer beams at the optical recombination point of less than about 5xc2x0 at 25xc2x0 C. after the temperature of the interferometric device has been cycled up to 80xc2x0 C. and returned back to 25xc2x0 C.
The two interferometer legs may comprise two legs in an arrayed waveguide configuration and may include at least one Bragg grating in the at least two interferometer legs.